High efficiency LED driving method

ABSTRACT

An arrangement wherein a plurality of LED strings are driven with a balanced drive signal, i.e. a drive signal wherein the positive side and negative side are of equal energy over time, is provided. In a preferred embodiment, the drive signal is balanced responsive to a capacitor provided between a switching network and a driving transformer. Balance of current between various LED strings is provided by a balancing transformer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/482,116 filed May 3, 2011, entitled “HighEfficiency LED Driving Method”, the entire contents of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of solid state lighting, andin particular to an LED driving arrangement with a balancer and acapacitively coupled driving signal.

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) have become very popular for use aslighting devices due to their advantages of high efficiency, long life,mechanical compactness and robustness, and low voltage operation,without limitation. Application areas include liquid crystal display(LCD) backlight, general lighting, and signage display. LEDs exhibitsimilar electrical characteristics to diodes, i.e. LEDs only conductcurrent when the forward voltage across the device reaches itsconduction threshold, denoted V_(F), and when the forward voltageincreases above V_(F) the current flowing through the device increasessharply. As a result a particular drive circuit has to be furnished inorder to control the LED current stably.

The existing approach in today's market normally uses a switching typeDC to DC converter, typically in a current control mode, to drive theLED lighting device. Because of the limited power capacity of a singleLED device, in most applications multiple LED's are connected in seriesto form a LED string, and multiple such LED strings work together,typically in parallel, to produce the desired light intensity. Inmultiple LED string applications a DC to DC converter is normallyemployed to supply a DC voltage sufficient for the LED operation,however because the operating voltage of LEDs have a wide tolerance(+/−5% to +/−10%), an individual control circuit has to be deployed witheach LED string to regulate its current. For simplicity, such a currentregulator typically employs a linear regulation technique, wherein apower regulation device is connected in series with the LED string andthe LED current is controlled by adjusting the voltage drop across thepower regulating device. Unfortunately, such an approach consumesexcessive power and generates excessive heat because of the powerdissipation of the linear regulation devices. In some approaches aswitching type DC to DC converter is provided for each LED string. Suchan approach yields a high efficiency operation but the associated costsalso increase dramatically.

What is needed, and not provided by the prior art, is an LED drivemethod with high operating efficiency and a low system cost, whichprovides a balancing function between the various LED strings of amultiple LED string luminaire.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention toovercome at least some of the disadvantages of the prior art. This isprovided in certain embodiments by an arrangement wherein a plurality ofLED strings are driven with a balanced drive signal, i.e. a drive signalwherein the positive side and negative side are forced to be of equalenergy over time. In a preferred embodiment, the drive signal isbalanced responsive to a capacitor provided between a switching networkand a driving transformer. Balance of current between various LEDstrings is provided by a balancing transformer.

Additional features and advantages of the invention will become apparentfrom the following drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect, reference will now be made, purely by way ofexample, to the accompanying drawings in which like numerals designatecorresponding elements or sections throughout.

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice. In the accompanying drawings:

FIG. 1 illustrates a high level schematic diagram of an embodiment of adriving arrangement for four LED strings wherein the anode end of eachof the LED strings are commonly coupled to the center tap of a drivingtransformer, and wherein the cathode ends of the LED strings are eachcoupled to respective ends of windings of a balancing transformer viarespective unidirectional electronic valves;

FIG. 2 illustrates a high level schematic diagram of an embodiment of adriving arrangement for four LED strings wherein the anode end of eachof the LED strings are commonly coupled to the center tap of a drivingtransformer, the cathode ends are each coupled to respective ends ofwindings of a balancing transformer, and the center taps of thebalancing transformer windings are coupled to the driving transformersecond winding ends via respective unidirectional electronic valves;

FIG. 3 illustrates a high level schematic diagram of an embodiment of adriving arrangement for two LED strings wherein the anode end of each ofthe LED strings are commonly coupled to the center tap of a drivingtransformer, the cathode ends of the LED strings are each coupled to acenter tap of respective windings of a balancing transformer, and thebalancing transformer winding ends are coupled to the drivingtransformer second winding ends via respective unidirectional electronicvalves;

FIG. 4 illustrates a high level schematic diagram of an embodiment of adriving arrangement for four LED strings wherein the cathode ends of afirst two of the LED strings are commonly coupled to a first end of thesecond winding of a driving transformer, the cathode ends of a secondtwo of the LED strings are commonly coupled to a second end of thesecond winding of the driving transformer, and the anode ends of the LEDstrings are each coupled to respective ends of windings of a balancingtransformer; and

FIG. 5 illustrates a high level schematic diagram of an embodiment of adriving arrangement for two LED strings wherein the cathode end of eachof the LED strings are commonly coupled to the center tap of a drivingtransformer, the anode ends of the LED strings are each coupled to acenter tap of respective windings of a balancing transformer, and thebalancing transformer winding ends are coupled to the drivingtransformer second winding ends via respective unidirectional electronicvalves.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is applicable to other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

FIG. 1 illustrates a high level schematic diagram of an embodiment of adriving arrangement 10 comprising: a switching control circuit 20; aswitching bridge 30 comprising a first electronically controlled switchQ1 and a second electronically controlled switch Q2; a DC blockingcapacitor CX; a driving transformer TX comprising a first winding TXFmagnetically coupled to a second winding TXS; first, second, third andfourth LED strings 40; a balancing transformer BX comprising a firstwinding BXF magnetically coupled to a second winding BXS; a first,second, third and fourth smoothing capacitors CS; and a first, second,third and fourth unidirectional electronic valve 50. First and secondelectronically controlled switches Q1, Q2 are illustrated withoutlimitation as NMOSFETs, however this is not meant to be limiting in anyway. Switching bridge 30 is illustrated as a half bridge, however thisis not meant to be limiting in any way, and in particular embodiment afull bridge is implemented without exceeding the scope.

A first output of switching control circuit 20, denoted VG1, is coupledto the control input of first electronically controlled switch Q1 ofswitching bridge 30, and a second output of switching control circuit20, denoted VG2, is coupled to the control input of secondelectronically controlled switch Q2 of switching bridge 30. The drain offirst electronically controlled switch Q1 is coupled to a source ofelectrical power, denoted V+, and the source of first electronicallycontrolled switch Q1 is coupled to drain of second electronicallycontrolled switch Q2 and to a first end of DC blocking capacitor CX. Thecommon node of the source of first electronically controlled switch Q1,the drain of second electronically controlled switch Q2, and the firstend of DC blocking capacitor CX is denoted node 35. The second end of DCblocking capacitor CX is coupled to a first end of first winding TXF,and a second end of first winding TXF is coupled to the source of secondelectronically controlled switch Q2, and to the return of the source ofelectrical power, denoted V−.

A center tap of second winding TXS is coupled to the anode end of eachof the LED strings 40 and to a first end of each of the smoothingcapacitors CS. The cathode end of each of the LED strings 40 is coupledto a second end of a respective smoothing capacitor CS and to the anodeof a respective unidirectional electronic valve 50. The cathode of afirst unidirectional electronic valve is coupled to a first end of firstwinding BXF, the cathode of a second unidirectional electronic valve 50is coupled to a second end of first winding BXF, the cathode of a thirdunidirectional electronic valve 50 is coupled to a first end of secondwinding BXS, and the cathode of a fourth unidirectional electronic valve50 is coupled to a second end of second winding BXS. A center tap offirst winding BXF is coupled to a first end of second winding TXS, and acenter tap of second winding BXS is coupled to a second end of secondwinding TXS.

In operation, and as will be described further below, drivingarrangement 10 provides a balanced current for 4 LED strings 40 with asingle balancing transformer BX. The 4 LED strings 40 are configuredwith a common anode structure. The balancing transformer BX has twocenter tapped windings, each of the two windings BXF and BXS having thesame number of turns. The center taps of BXF, BXS and TXS are eachpreferably arranged such that an equal number of turns are exhibitedbetween the center tap and the respective opposing ends of the winding.

Switching control circuit 20 is arranged to alternately close firstelectronically controlled switch Q1 and second electronically controlledswitch Q2 so as to provide a switching cycle having a first periodduring which electrical energy is output from second winding TXS with afirst polarity and a second period during which electrical energy isoutput from second winding TXS with a second polarity, the secondpolarity opposite the first polarity.

During the first period, when the end of second winding TXS coupled tothe center tap of first winding BXF is negative in relation to thecenter tap of second winding TXS, current flows through the two LEDstrings 40 coupled to the respective ends of first winding BXF. Duringthe second period, when the end of second winding TXS coupled to thecenter tap of second winding BXS is negative in relation to the centertap of second winding TXS, current flows through the two LED strings 40coupled to the respective ends of second winding BXS. The currentthrough the two LED strings 40 conducting during the first period areforced to be equal by the balancing effect of the two winding halves offirst winding BXF, and current through the two LED strings 40 conductingduring the second period are forced to be equal by the balancing effectof the two winding halves of second winding BXS. DC blocking capacitorCX ensures that the current flowing through first winding TXF, and hencetransferred to second winding TXS, during each of the two periods isequal, because DC blocking capacitor CX does not couple DC current insteady state. In the event that the average operating voltage of the twoLED strings 40 coupled to first winding BXF is different than theaverage operating voltage of the two LED strings 40 coupled to secondwinding BXS, a DC bias will automatically develop across DC blockingcapacitor CX to offset the average operating voltage difference. The DCbias acts to maintain an equal total current for each of the two stringgroups, i.e. the first group comprising two LED strings 40 coupled tofirst winding BXF and the second group comprising two LED strings 40coupled to second winding BXS.

To further clarify and illustrate this relationship, we denote thecurrent through the two LED strings 40 coupled to first winding BXF,respectively, as I_(LED1) and I_(LED2). We further denote the currentthrough the two LED strings 40 coupled to second winding BXS,respectively, as I_(LED3) and I_(LED4). This results in the followingrelations.I_(LED1)+I_(LED2)=I_(LED3)+I_(LED4) (Responsive to CX)   EQ. 1I_(LED1)=I_(LED2), I_(LED3)=I_(LED4) (Responsive to BX)   EQ. 2And as result of EQ. 1 and EQ. 2: I_(LED1)=I_(LED2)=I_(LED3)=I_(LED4)

Smoothing capacitors CS are each connected in parallel with a respectiveone of LED strings 40 to smooth out any ripple current and maintain theassociated LED current to be nearly a constant direct current.Unidirectional electronic valves 50 are arranged to block any reversevoltage to LED strings 40 and further prevent bleeding of currentbetween respective smoothing capacitors CS.

FIG. 2 illustrates a high level schematic diagram of an embodiment of adriving arrangement 100 for four LED strings 40, wherein the anode endof each LED string 40 is commonly coupled to the center tap of secondwinding TXS of driving transformer TX, the cathode ends of the variousLED strings 40 are each coupled to respective ends of windings ofbalancing transformer BX, and the center taps of the balancingtransformer windings, BXS and BXF, are coupled to driving transformersecond winding TXS via respective unidirectional electronic valves 50.Driving arrangement 100 is a simplified version of driving arrangement10, wherein LED strings 40 are allowed to operate with a rippledcurrent, and thus smoothing capacitors CS are not supplied and only asingle unidirectional electronic valve 50 is required for each two LEDstrings 40.

In some further detail, the center tap of second winding TXS is commonlycoupled to the anode end of each of the four LED strings 40. The cathodeend of first LED string 40 is coupled to a first end of first windingBXF; the cathode end of second LED string 40 is coupled to a second endof first winding BXF; the cathode end of third LED string 40 is coupledto a first end of second winding BXS; and the cathode end of fourth LEDstring 40 is coupled to a second end of second winding BXS. The centertap of first winding BXF is coupled via a respective unidirectionalelectronic valve 50 to a first end of second winding TXS and the centertap of second winding BXS is coupled via a respective unidirectionalelectronic valve 50 to a second end of second winding TXS. Switchingcontrol circuit 20 is not shown for simplicity, and the connections ofswitching bridge 30, DC blocking capacitor CX and first winding TXF areas described above in relation to driving arrangement 10.

The operation of driving arrangement 100 is in all respects similar tothe operation of driving arrangement 10, and thus in the interest ofbrevity will not be further detailed.

FIG. 3 illustrates a high level schematic diagram of an embodiment of adriving arrangement 200 having two LED strings 40. Switching controlcircuit 20 is not shown for simplicity, and the connections of switchingbridge 30, DC blocking capacitor CX and first winding TXF are asdescribed above in relation to driving arrangement 10. The anode end ofeach of the LED strings 40 are commonly coupled to the center tap ofsecond winding TXS of driving transformer TX. The cathode end of a firstLED string 40 is coupled to a center tap of first winding BXF ofbalancing transformer BX, and the cathode end of a second LED string 40is coupled to a center tap of second winding BXS of balancingtransformer BX. The ends of first winding BXF are each coupled via arespective unidirectional electronic valve 50 to respective ends ofsecond winding TXS of driving transformer TX and respective ends ofsecond winding BXF are each coupled via a respective unidirectionalelectronic valve 50 to respective ends of second winding TXS of drivingtransformer TX.

Each winding of balancing transformer BX thus drives a single LED string40. The LED strings 40 each conduct in both half cycles and thereforethe ripple current frequency is twice that of the switching frequency ofQ1 and Q2. Opposing halves of first winding BXF conduct during therespective first and second periods generated by switching controlcircuit 20 and opposing halves of second winding BXS conduct during therespective first and second periods generated by switching controlcircuit 20 (not shown). Therefore the core of balancer transformer BXexperiences an AC excitation. The connection polarity of balancerwindings BXF and BXS is such so as to always keep the magnetizationforce generated by the current of the two LED strings 40 in oppositedirections, and by such magnetization force the current of the two LEDstrings 40 are forced to be equal.

Driving arrangements 10, 100 and 200 illustrate a common anode structurefor LED strings 40, however this is not meant to be limiting in any way,as will be further illustrated below.

FIG. 4 illustrates a high level schematic diagram of an embodiment of adriving arrangement 300 exhibiting four LED strings 40. Switchingcontrol circuit 20 is not shown for simplicity, and the connections ofswitching bridge 30, DC blocking capacitor CX and first winding TXF areas described above in relation to driving arrangement 10. The cathodeends of a first two LED strings 40 are commonly coupled to a first endof second winding TXS of driving transformer TX via a common respectiveunidirectional electronic valve 50 and the cathode ends of a second twoLED strings 40 are commonly coupled to a second end of second windingTXS of driving transformer TX via a common respective unidirectionalelectronic valve 50. The anode end of first LED string 40 is coupled toa first end of first winding BXF of balancing transformer BS; the anodeend of second LED string 40 is coupled to a second end of first windingBXF of balancing transformer BS; the anode end of third LED string 40 iscoupled to a first end of second winding BXS of balancing transformerBS; and the anode end of fourth LED string 40 is coupled to a second endof second winding BXS of balancing transformer BS. The center taps ofeach of first winding BXF and second winding BXS are commonly coupled tothe center tap of second winding TXS of driving transformer TX.

The operation of driving arrangement 300 is in all respects similar tothe operation of driving arrangement 100, with first and second LED 40providing illumination during one of the first and second periods, andthe third and fourth LED 40 providing illumination during the other ofthe first and second periods, and in the interest of brevity will not bedetailed further.

FIG. 5 illustrates a high level schematic diagram of an embodiment of adriving arrangement 400 for two LED strings 40 wherein the cathode endof each of the LED strings 40 are commonly coupled to the center tap ofsecond winding TXS of driving transformer TX. Switching control circuit20 is not shown for simplicity, and the connections of switching bridge30, DC blocking capacitor CX and first winding TXF are as describedabove in relation to driving arrangement 10. The anode end of first LEDstring 40 is coupled to the center tap of first winding BXF of balancingtransformer BX and the anode end of second LED string 40 is coupled tothe center tap of second winding BXS of balancing transformer BX. Afirst end of first winding BXF is coupled via a respectiveunidirectional electronic valve 50 to a first end of second winding TXSof driving transformer TX; a second end of first winding BXF is coupledvia a respective unidirectional electronic valve 50 to a second end ofsecond winding TXS of driving transformer TX; a first end of secondwinding BXS is coupled via a respective unidirectional electronic valve50 to a first end of second winding TXS of driving transformer TX; and asecond end of second winding BXS is coupled via a respectiveunidirectional electronic valve 50 to a second end of second winding TXSof driving transformer TX.

The operation of driving arrangement 400 are in all respects identicalwith the operation of driving arrangement 200, with the appropriatechanges in polarity as required, and thus in the interest of brevitywill not be further detailed.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable sub-combination.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meanings as are commonly understood by one of ordinaryskill in the art to which this invention belongs. Although methodssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods aredescribed herein.

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Incase of conflict, the patent specification, including definitions, willprevail. In addition, the materials, methods, and examples areillustrative only and not intended to be limiting.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. Rather the scope of the present invention isdefined by the appended claims and includes both combinations andsub-combinations of the various features described hereinabove as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot in the prior art.

I claim:
 1. A driving arrangement for light emitting diode (LED) basedluminaire comprising: a driving transformer having a first winding and asecond winding, the second winding magnetically coupled to the firstwinding; a switching control circuit; a switching bridge comprising apair of electronically controlled switches coupled to a common node,each of the pair of electronically controlled switches responsive to anoutput of the switching control circuit, said common node coupled to afirst end of the primary winding of the driving transformer; a directcurrent (DC) blocking capacitor coupled between the common node of saidswitching bridge and a first end of in series with the primary windingof the driving transformer; a balancing transformer having a firstwinding and a second winding, the second winding magnetically coupled tothe first winding; a first LED string; and a second LED string, a firstend of each of said first LED string and said second LED string coupledto the second winding of said driving transformer, and arranged toreceive electrical energy there from; and a second end of said first LEDstring coupled to a first winding of said balancing transformer, and asecond end of said second LED string coupled to a second winding of saidbalancing transformer, said switching control circuit arranged toprovide a switching cycle comprising a first period wherein electricalenergy is output from the second winding of said driving transformerwith a first polarity, and a second period wherein electrical energy isoutput from the second winding of said driving transformer with a secondpolarity, the second polarity opposite the first polarity, said DCblocking capacitor arranged such that the total electrical energy outputfrom the second winding during the first period of the switching cycleis equal to the total electrical energy output from the second windingduring the second period of the switching cycle, and said balancingtransformer arranged such that the current through said first LED stringis equal to the current through said second LED string.
 2. The drivingarrangement according to claim 1, wherein: the first end of each of saidfirst LED string and said second LED string are commonly coupled to acenter tap of the second winding of said driving transformer; the secondend of said first LED string coupled to a center tap of said firstwinding of the balancing transformer; and the second end of said secondLED string coupled to a center tap of said second winding of thebalancing transformer.
 3. The driving arranged according to claim 2,wherein: a first end of the first winding of the balancing transformeris coupled to a first end of the second winding of the drivingtransformer; a second end of the first winding of the balancingtransformer is coupled to a second end of the second winding of thedriving transformer; a first end of the second winding of the balancingtransformer is coupled to the second end of the second winding of thedriving transformer; and a second end of the second winding of thebalancing transformer is coupled to the first end of the second windingof the driving transformer.
 4. The driving arrangement according toclaim 2, further comprising: a first, second, third and fourthunidirectional electronic valve, wherein: a first end of the firstwinding of the balancing transformer is coupled to a first end of thesecond winding of the driving transformer via said first unidirectionalelectronic valve; a second end of the first winding of the balancingtransformer is coupled to a second end of the second winding of thedriving transformer via said second unidirectional electronic valve; afirst end of the second winding of the balancing transformer is coupledto the second end of the second winding of the driving transformer viasaid third unidirectional electronic valve; and a second end of thesecond winding of the balancing transformer is coupled to the first endof the second winding of the driving transformer via said fourthunidirectional electronic valve.
 5. The driving arrangement according toclaim 1, further comprising: a third LED string; and a fourth LEDstring, wherein: a first end of each of said third LED string and saidfourth LED string are coupled to the second winding of said drivingtransformer, and arranged to receive electrical energy there from; asecond end of said third LED string is coupled to the first winding ofsaid balancing transformer; and a second end of said fourth LED stringis coupled to the second winding of said balancing transformer, saidfirst winding of said balancing transformer arranged such that thecurrent through said first LED string is equal to the current throughsaid third LED string, and said second winding of said balancingtransformer arranged such that the current through said second LEDstring is equal to the current through said fourth LED string.
 6. Thedriving arrangement according to claim 5, wherein: the first end of eachof the first LED string, the second LED string, the third LED string andthe fourth LED string are commonly coupled a center tap of the secondwinding of said driving transformer; the second end of said first LEDstring is coupled to a first end of the first winding of said balancingtransformer; the second end of said second LED string is coupled to afirst end of the second winding of said balancing transformer; thesecond end of said third LED string is coupled to a second end of thefirst winding of said balancing transformer; the second end of saidfourth LED string is coupled to a second end of the second winding ofsaid balancing transformer; a first end of the second winding of saiddriving transformer is coupled to a center tap of the first winding ofthe balancing transformer; and a second end of the second winding ofsaid driving transformer is coupled to a center tap of the secondwinding of the balancing transformer.
 7. The driving arrangementaccording to claim 6, further comprising: a first, a second, a third anda fourth unidirectional electronic valve, wherein: the second end ofsaid first LED string is coupled to the first end of the first windingof said balancing transformer via said first unidirectional electronicvalve; the second end of said second LED string is coupled to the firstend of the second winding of said balancing transformer via said secondunidirectional electronic valve; the second end of said third LED stringis coupled to the second end of the first winding of said balancingtransformer via said third unidirectional electronic valve; and thesecond end of said fourth LED string is coupled to the second end of thesecond winding of said balancing transformer via said fourthunidirectional electronic valve.
 8. The driving arrangement according toclaim 6, further comprising: a first and a second unidirectionalelectronic valve, wherein: the first end of the second winding of saiddriving transformer is coupled to the center tap of the first winding ofthe balancing transformer via said first unidirectional electronicvalve; and the second end of the second winding of said drivingtransformer is coupled to the center tap of the second winding of thebalancing transformer via said second unidirectional electronic valve.9. The driving arrangement according to claim 5, wherein: the first endof each of the first LED string and the third LED string are coupled toa first end of the second winding of the driving transformer; the firstend of each of the second LED string and the fourth LED string arecoupled to the second of the second winding of the driving transformer;the second end of the first LED string coupled to a first end of thefirst winding of the balancing transformer; the second end of the secondLED string coupled to a first end of the second winding of the balancingtransformer; the second end of the third LED string coupled to a secondend of the first winding of the balancing transformer; and the secondend of the fourth LED string coupled to a second end of the secondwinding of the balancing transformer.
 10. The driving arrangement ofclaim 9, further comprising: a first and a second unidirectionalelectronic valve, wherein: the first end of the first and third LEDstrings are coupled to the first end of the second winding of thedriving transformer via said first unidirectional electronic valve; andthe first end of the second and fourth LED strings are coupled to thesecond end of the second winding of the driving transformer via saidsecond unidirectional electronic valve.
 11. A driving arrangement forlight emitting diode (LED) based luminaire comprising: a means fordriving having a first winding and a second winding, the second windingmagnetically coupled to the first winding; a means for switching; aswitching bridge comprising a pair of electronically controlled switchescoupled to a common node, each of the pair of electronically controlledswitches responsive to an output of the means for switching, said commonnode coupled to a first end of the first winding of the means fordriving; a direct current (DC) blocking capacitor coupled between thecommon node of said switching bridge and a first end of the primary inseries with the first winding of the means for driving; a balancingtransformer having a first winding and a second winding, the secondwinding magnetically coupled to the first winding; a first LED string;and a second LED string, a first end of each of said first LED stringand said second LED string coupled to the second winding of said meansfor driving, and arranged to receive electrical energy there from; and asecond end of said first LED string coupled to a first winding of saidbalancing transformer, and a second end of said second LED stringcoupled to a second winding of said balancing transformer, said meansfor switching arranged to provide a switching cycle comprising a firstperiod wherein electrical energy is output from the second winding ofsaid means for driving with a first polarity, and a second periodwherein electrical energy is output from the second winding of saidmeans for driving with a second polarity, the second polarity oppositethe first polarity, said DC blocking capacitor arranged such that thetotal electrical energy output from the second winding during the firstperiod of the switching cycle is equal to the total electrical energyoutput from the second winding during the second period of the switchingcycle, and said balancing transformer arranged such that the currentthrough said first LED string is equal to the current through saidsecond LED string.
 12. The driving arrangement according to claim 11,wherein: the first end of each of said first LED string and said secondLED string are commonly coupled to a center tap of the second winding ofsaid means for driving; the second end of said first LED string coupledto a center tap of said first winding of the balancing transformer; andthe second end of said second LED string coupled to a center tap of saidsecond winding of the balancing transformer.
 13. The driving arrangedaccording to claim 12, wherein: a first end of the first winding of thebalancing transformer is coupled to a first end of the second winding ofthe means for driving; a second end of the first winding of thebalancing transformer is coupled to a second end of the second windingof the means for driving; a first end of the second winding of thebalancing transformer is coupled to the second end of the second windingof the means for driving; and a second end of the second winding of thebalancing transformer is coupled to the first end of the second windingof the means for driving.
 14. The driving arrangement according to claim12, further comprising: a first, second, third and fourth unidirectionalelectronic valve, wherein: a first end of the first winding of thebalancing transformer is coupled to a first end of the second winding ofthe means for driving via said first unidirectional electronic valve; asecond end of the first winding of the balancing transformer is coupledto a second end of the second winding of the means for driving via saidsecond unidirectional electronic valve; a first end of the secondwinding of the balancing transformer is coupled to the second end of thesecond winding of the means for driving via said third unidirectionalelectronic valve; and a second end of the second winding of thebalancing transformer is coupled to the first end of the second windingof the means for driving via said fourth unidirectional electronicvalve.
 15. The driving arrangement according to claim 11, furthercomprising: a third LED string; and a fourth LED string, wherein: afirst end of each of said third LED string and said fourth LED stringare coupled to the second winding of said means for driving, andarranged to receive electrical energy there from; a second end of saidthird LED string is coupled to the first winding of said balancingtransformer; and a second end of said fourth LED string is coupled tothe second winding of said balancing transformer, said first winding ofsaid balancing transformer arranged such that the current through saidfirst LED string is equal to the current through said third LED string,and said second winding of said balancing transformer arranged such thatthe current through said second LED string is equal to the currentthrough said fourth LED string.
 16. The driving arrangement according toclaim 15, wherein: the first end of each of the first LED string, thesecond LED string, the third LED string and the fourth LED string arecommonly coupled a center tap of the second winding of said means fordriving; the second end of said first LED string is coupled to a firstend of the first winding of said balancing transformer; the second endof said second LED string is coupled to a first end of the secondwinding of said balancing transformer; the second end of said third LEDstring is coupled to a second end of the first winding of said balancingtransformer; the second end of said fourth LED string is coupled to asecond end of the second winding of said balancing transformer; a firstend of the second winding of said means for driving is coupled to acenter tap of the first winding of the balancing transformer; and asecond end of the second winding of said means for driving is coupled toa center tap of the second winding of the balancing transformer.
 17. Thedriving arrangement according to claim 16, further comprising: a first,a second, a third and a fourth unidirectional electronic valve, wherein:the second end of said first LED string is coupled to the first end ofthe first winding of said balancing transformer via said firstunidirectional electronic valve; the second end of said second LEDstring is coupled to the first end of the second winding of saidbalancing transformer via said second unidirectional electronic valve;the second end of said third LED string is coupled to the second end ofthe first winding of said balancing transformer via said thirdunidirectional electronic valve; and the second end of said fourth LEDstring is coupled to the second end of the second winding of saidbalancing transformer via said fourth unidirectional electronic valve.18. The driving arrangement according to claim 16, further comprising: afirst and a second unidirectional electronic valve, wherein: the firstend of the second winding of said means for driving is coupled to thecenter tap of the first winding of the balancing transformer via saidfirst unidirectional electronic valve; and the second end of the secondwinding of said means for driving is coupled to the center tap of thesecond winding of the balancing transformer via said secondunidirectional electronic valve.
 19. The driving arrangement accordingto claim 15, wherein: the first end of each of the first LED string andthe third LED string are coupled to a first end of the second winding ofthe means for driving; the first end of each of the second LED stringand the fourth LED string are coupled to the second of the secondwinding of the means for driving; the second end of the first LED stringcoupled to a first end of the first winding of the balancingtransformer; the second end of the second LED string coupled to a firstend of the second winding of the balancing transformer; the second endof the third LED string coupled to a second end of the first winding ofthe balancing transformer; and the second end of the fourth LED stringcoupled to a second end of the second winding of the balancingtransformer.
 20. The driving arrangement of claim 19, furthercomprising: a first and a second unidirectional electronic valve,wherein: the first end of the first and third LED strings are coupled tothe first end of the second winding of the means for driving via saidfirst unidirectional electronic valve; and the first end of the secondand fourth LED strings are coupled to the second end of the secondwinding of the means for driving via said second unidirectionalelectronic valve.